Recently, modular multilevel converters are gaining much attention in industry applications and academics. The main features of modular multilevel converters are reduced harmonics, lower switching frequency, and reduced stress on each device.
The modular multilevel converters have been used widely for a power application such as high voltage direct current (HVDC), flexible alternating current transmission system (FACTS), high/medium power drives application, and the like. The modular multilevel converter topology provides a modular structure and can be extended to any power requirements. The modular multilevel converter topology can be classified into two major categories: the modular multilevel voltage source converter and the modular multilevel current source converter.
The modular multilevel voltage source converter topology can increase the number of voltage levels by increasing the sub-module number. This will increase the number of switches required. Normally, this type of multilevel converter topology may obtain one or two extra voltage levels in the output level by increasing one sub-module. Hence, to get more number of levels in the output voltage, the large number of sub-modules is required. This results in increase in the control complexity, losses and cost of converter circuits.
Similarly, in the existing modular multilevel current source converter circuit topology, the number of output current levels is N+1 for N half bridge modules per arm and by increasing one half bridge module in an arm, one extra current level in the output is obtained. Hence, with the increase of the number of the output current level, the corresponding need to increase the number of modules makes the entire circuit very large and control may also become more complex. Reducing the number of the modules for the same number of output current level reduces costs and simplifies the control, while increasing the reliability of the system.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.